Two different power supplies are connected in turn to an oscilloscope. The traces obtained are shown in Fig. 12.1. (a) State which trace, A or B, shows a di...
Question 1 Report
Two different power supplies are connected in turn to an oscilloscope. The traces obtained are shown in Fig. 12.1.
(a) State which trace, A or B, shows a direct current (d.c.) supply. [1]
(b) Record the voltage of the d.c. supply shown by trace A, using the Y-gain setting given. [1]
(c) State the peak voltage of the supply shown by trace B, using the Y-gain setting given. [1]
(d) Describe the key difference between the two types of current shown by these traces. [1]
(e) Suggest one practical source of the supply that produces trace A. [1]
(f) Suggest one practical source of the supply that produces trace B. [1]
(g) Draw on Fig. 12.1 what trace B would look like if the frequency of the a.c. supply were doubled. [1]
Answer Details
(a) Trace A shows the d.c. supply. A d.c. trace appears as a straight horizontal line on the oscilloscope screen because the voltage has a constant value that does not change with time. [1]
(b) The d.c. line sits about 1.25 divisions above the centre line. With the Y-gain set to 2.0 V per division:
\[ V = 1.25 \times 2.0 = 2.5\;\text{V} \] [1]
(c) The a.c. sine wave in Trace B peaks about 2 divisions above (and below) the centre line. The peak voltage is:
\[ V_{\text{peak}} = 2 \times 2.0 = 4.0\;\text{V} \] [1]
(d) Direct current has a constant value and flows in one direction only, producing a flat horizontal trace. Alternating current continuously changes magnitude and periodically reverses direction, producing a sine-wave trace that oscillates above and below the centre line. [1]
(e) A battery, a cell, or a d.c. power pack. These supply a steady voltage in one direction. [1]
(f) The mains electricity supply, a signal generator, or an a.c. power supply. These produce an alternating voltage. [1]
(g) Doubling the frequency means twice as many complete cycles fit in the same screen width. The amplitude (peak height) stays the same, but each wave is compressed to half the horizontal width. The trace would show four complete cycles instead of two across the screen. [1]
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